An invariant measure δ is introduced to quantify the disorder in extended locally striped patterns. It is invariant under Euclidean motions of the pattern, and vanishes for a uniform array of stripes. Irregularities such as point defects and domain walls make nonzero contributions to the measure. The evolution of random initial states to labyrinthine patterns is analyzed through the time evolution of δ. This behavior is configuration independent, and exhibits two phases each with a logarithmic decay in δ.